Control of Elemental Distribution in the Nanoscale Solid-State Reaction That Produces (Ga1-xZnx)(N1-xOx) Nanocrystals.

Solid-state chemical transformations at the nanoscale can be a powerful tool for achieving compositional complexity in nanomaterials. It is desirable to understand the mechanisms of such reactions and characterize the local-level composition of the resulting materials. Here, we examine how reaction temperature controls the elemental distribution in (Ga1-xZnx)(N1-xOx) nanocrystals (NCs) synthesized via the solid-state nitridation of a mixture of nanoscale ZnO and ZnGa2O4 NCs. (Ga1-xZnx)(N1-xOx) is a visible-light absorbing semiconductor that is of interest for applications in solar photochemistry. We couple elemental mapping using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) with colocation analysis to study the elemental distribution and the degree of homogeneity in the (Ga1-xZnx)(N1-xOx) samples synthesized at temperatures ranging from 650 to 900 °C with varying ensemble compositions (i.e., x values). Over this range of temperatures, the elemental distribution ranges from highly heterogeneous at 650 °C, consisting of a mixture of larger particles with Ga and N enrichment near the surface and very small NCs, to uniform particles with evenly distributed constituent elements for most compositions at 800 °C and above. We propose a mechanism for the formation of the (Ga1-xZnx)(N1-xOx) NCs in the solid state that involves phase transformation of cubic spinel ZnGa2O4 to wurtzite (Ga1-xZnx)(N1-xOx) and diffusion of the elements along with nitrogen incorporation. The temperature-dependence of nitrogen incorporation, bulk diffusion, and vacancy-assisted diffusion processes determines the elemental distribution at each synthesis temperature. Finally, we discuss how the visible band gap of (Ga1-xZnx)(N1-xOx) NCs varies with composition and elemental distribution

Efficient Photocatalytic Hydrogen Generation from Ni Nanoparticle Decorated CdS Nanosheets

High-quality, thickness-controlled CdS nanosheets (NSs) have been obtained through the thermal decomposition of cadmium diethyldithiocarbamate in octadecene. Ensembles with discrete thicknesses of 1.50, 1.80, and 2.16 nm have been made with corresponding lateral dimensions on the order of 90 nm × 20 nm. These latter values make the 1–3 nm NSs the largest 2D CdS specimens made to date using colloidal chemistry. Associated Ni nanoparticle decorated counterparts have been made through the photodeposition of Ni onto NSs with an average nanoparticle diameter of 6 nm. Subsequent photocatalytic hydrogen generation measurements have compared the performance of CdS NSs with that of their Ni NP decorated counterparts in water/ethanol mixtures. Apparent quantum yields as large as 25% have been seen for Ni NP decorated NSs with transient yields as large as 64% within the first 2 h of irradiation. Results from ensemble femtosecond transient differential absorption spectroscopy reveal that the origin of this high efficiency stems from efficient electron transfer from CdS to Ni. In this regard, the CdS/Ni semiconductor/metal heterojunction acts to dissociate strongly bound excitons in CdS NSs, creating free carriers needed to carry out relevant reduction chemistries

Synthesis of Ultrathin and Thickness-Controlled Cu_2–<i>x</i>Se Nanosheets via Cation Exchange

We demonstrate the use of cation exchange to synthesize ultrathin and thickness-controlled Cu_2–<i>x</i>Se nanosheets (NSs) beginning with CdSe NSs. In this manner, extremely thin (i.e., 1.6 nm thickness) Cu_2–<i>x</i>Se NSs, beyond which can be made directly, have been obtained. Furthermore, they represent the thinnest NSs produced via cation exchange. Notably, the exchange reaction preserves the starting morphology of the CdSe sheets and also retains their cubic crystal structure. The resulting nonstoichiometric and cubic Cu_2–<i>x</i>Se NSs are stable and do not exhibit any signs of Cu or Se oxidation after exposure to air for 2 weeks. Resulting NSs also show the existence of a localized surface plasmon resonance in the infrared due to the presence of copper vacancies. Efforts to isolate intermediates during the cation exchange reaction show that it occurs via a mechanism where entire sheets are rapidly converted into the final product once the exchange reaction commences, precluding the isolation of alloyed species

Spectroscopic signatures of ligand field states in {RuII(imine)} complexes

Ligand field (LF) states have been present in discussions on the photophysics and photochemistry of ruthenium-iminic chromophores for decades, although there is very little documented direct evidence of them. We studied the picosecond transient absorption (TA) spectroscopy of four {RuII(imine)} complexes that respond to the formula trans-[Ru(L)4(X)2], where L is either pyridine (py) or 4-methoxypyridine (MeOpy) and X is either cyanide or thiocyanate. Dicyano compounds behave as most ruthenium polypyridines and their LF states remain silent. In contrast, in the dithiocyanate complexes we found clear spectroscopic evidence of the participation of LF states in the MLCT decay pathway. These states are of donor and acceptor character simultaneously and this is manifested in the presence of MLCT and LMCT transient absorption bands of similar energy. Spectroelectrochemical techniques supported the interpretation of the absorption features of MLCT states, and DFT methods helped to assign their spectroscopic signatures and provided strong evidence on the nature of LF states.Fil: Cadranel, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Pieslinger, German Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Tongying, Pornthip. University of Notre Dame; Estados UnidosFil: Kuno, Masaru K.. University of Notre Dame; Estados UnidosFil: Baraldo Victorica, Luis Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Hodak, Jose Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

Synthetic Strategy and Structural and Optical Characterization of Thin Highly Crystalline Titanium Disulfide Nanosheets

Two-dimensional (2D) nanomaterials have recently received significant attention because of their attractiveness for use in many nanostructured devices. Layered transition-metal dichalcogenides are of particular interest because reducing their dimensionality causes changes in their already anisotropic physical and chemical properties. The present study describes the first bottom-up solution-phase synthesis of thin highly crystalline titanium disulfide (TiS₂) nanosheets (NSs) using abundant low-cost molecular precursors. The obtained TiS₂ NSs have average dimensions of ∼500 nm × 500 nm in the basal plane and have thicknesses of ∼5 nm. They exhibit broad absorption in the visible that tails out into the near-infrared. The obtained results demonstrate new opportunities in synthesizing low-dimensional 2D nanomaterials with potential use in various photochemical energy applications

Photocatalytic Hydrogen Generation Efficiencies in One-Dimensional CdSe Heterostructures

To better understand the role nanoscale heterojunctions play in the photocatalytic generation of hydrogen, we have designed several model one-dimensional (1D) heterostructures based on CdSe nanowires (NWs). Specifically, CdSe/CdS core/shell NWs and Au nanoparticle (NP)-decorated core and core/shell NWs have been produced using facile solution chemistries. These systems enable us to explore sources for efficient charge separation and enhanced carrier lifetimes important to photocatalytic processes. We find that visible light H₂ generation efficiencies in the produced hybrid 1D structures increase in the order CdSe < CdSe/Au NP < CdSe/CdS/Au NP < CdSe/CdS with a maximum H₂ generation rate of 58.06 ± 3.59 μmol h^–1 g^–1 for CdSe/CdS core/shell NWs. This is 30 times larger than the activity of bare CdSe NWs. Using femtosecond transient differential absorption spectroscopy, we subsequently provide mechanistic insight into the role nanoscale heterojunctions play by directly monitoring charge flow and accumulation in these hybrid systems. In turn, we explain the observed trend in H₂ generation rates with an important outcome being direct evidence for heterojunction-influenced charge transfer enhancements of relevant chemical reduction processes

Relationships between Compositional Heterogeneity and Electronic Spectra of (Ga_1–<i>x</i>Zn_<i>x</i>)(N_1–<i>x</i>O_<i>x</i>) Nanocrystals Revealed by Valence Electron Energy Loss Spectroscopy

Many ternary and quaternary semiconductors have been made in nanocrystalline forms for a variety of applications, but we have little understanding of how well their ensemble properties reflect the properties of individual nanocrystals. We examine electronic structure heterogeneities in nanocrystals of (Ga1–xZnx)(N1–xOx), a semiconductor that splits water under visible illumination. We use valence electron energy loss spectroscopy (VEELS) in a scanning transmission electron microscope to map out electronic spectra of (Ga1–xZnx)(N1–xOx) nanocrystals with a spatial resolution of 8 nm. We examine three samples with varying degrees of intraparticle and interparticle compositional heterogeneity and ensemble optical spectra that range from a single band gap in the visible to two band gaps, one in the visible and one in the UV. The VEELS spectra resemble the ensemble absorption spectra for a sample with a homogeneous elemental distribution and a single band gap and, more interestingly, one with intraparticle compositional heterogeneity and two band gaps. We observe spatial variation in VEELS spectra only with significant interparticle compositional heterogeneity. Hence, we reveal the conditions under which the ensemble spectra reveal the optical properties of individual (Ga1–xZnx)(N1–xOx) particles. More broadly, we illustrate how VEELS can be used to probe electronic heterogeneities in compositionally complex nanoscale semiconductors

Control of Elemental Distribution in the Nanoscale Solid-State Reaction That Produces (Ga_1–<i>x</i>Zn_<i>x</i>)(N_1–<i>x</i>O_<i>x</i>) Nanocrystals

Solid-state chemical transformations at the nanoscale can be a powerful tool for achieving compositional complexity in nanomaterials. It is desirable to understand the mechanisms of such reactions and characterize the local-level composition of the resulting materials. Here, we examine how reaction temperature controls the elemental distribution in (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) nanocrystals (NCs) synthesized <i>via</i> the solid-state nitridation of a mixture of nanoscale ZnO and ZnGa₂O₄ NCs. (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) is a visible-light absorbing semiconductor that is of interest for applications in solar photochemistry. We couple elemental mapping using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) with colocation analysis to study the elemental distribution and the degree of homogeneity in the (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) samples synthesized at temperatures ranging from 650 to 900 °C with varying ensemble compositions (<i>i.e.</i>, <i>x</i> values). Over this range of temperatures, the elemental distribution ranges from highly heterogeneous at 650 °C, consisting of a mixture of larger particles with Ga and N enrichment near the surface and very small NCs, to uniform particles with evenly distributed constituent elements for most compositions at 800 °C and above. We propose a mechanism for the formation of the (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) NCs in the solid state that involves phase transformation of cubic spinel ZnGa₂O₄ to wurtzite (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) and diffusion of the elements along with nitrogen incorporation. The temperature-dependence of nitrogen incorporation, bulk diffusion, and vacancy-assisted diffusion processes determines the elemental distribution at each synthesis temperature. Finally, we discuss how the visible band gap of (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) NCs varies with composition and elemental distribution

Transforming Layered to Nonlayered Two-Dimensional Materials: Cation Exchange of SnS₂ to Cu₂SnS₃

We demonstrate the chemical transformation of layered, two-dimensional (2D) SnS₂ to nonlayered Cu₂SnS₃ via cation exchange. Resulting Cu₂SnS₃ nanosheets (NSs) retain the overall starting morphology of their parent, few-layer SnS₂ templates. Specifically, they possess micrometer-sized dimensions and have controlled thicknesses dictated by the number of initial SnS₂ layers. Our demonstration shows that existing layered compounds can serve as templates for difficult-to-synthesize nonlayered 2D specimens with cation exchange providing a bridge between families of layered and nonlayered materials. New 2D systems are therefore accessible, opening the door to future explorations of low-dimensional nanostructure anisotropic optical and electrical properties